I would guess that it would depend very much upon the actual
population, with some modification for strategic importance and other
factors.
A system with a 20 000 people might have one or two small
SDBs, and a small jump-capable ship based elsewhere that passes
through every now and then.
A system with 20 000 000 000 people
probably has a trillion credit capital fleet and I would expect a
naval base. The stated lack of one should indicate some unusual
political situation.